Bug 26327: Add ->library method to Koha::*Checkout

Bug 26327: Add ->library method to Koha::*Checkout

This patch adds a 'library' relationship to the Issue and OldIssue
resultsets, to be used in the Koha::*Checkout classes corresponding
accessors. This way we will be able to eventually embed and filter on
library fields on the API.

The ->library method is then added to the classes-

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Old.t t/db_dependent/Koha/Checkout.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>